Effect of flow direction on axial solid dispersion in gas-solids cocurrent upflow and downflow systems

Studies on the axial solid mixing mechanisms in gas-solids cocurrent upflow and downflow circulating fluidized bed systems have revealed that, among the many influencing factors, flow direction has the most profound influence on the axial solids mixing. When the flow is in the direction of gravity (downflow in the downer), axial solids dispersion is very small and the flow pattern approaches plug flow; when the flow is against gravity (upflow in the riser), axial solids dispersion is significantly larger and the flow pattern deviates significantly from plug flow. Solids mixing is found to be mainly due to the dispersion of dispersed particles in the downer while two solids mixing mechanisms co-exist in the riser: the dispersion of dispersed particles and the dispersion of particle clusters. Dispersion due to dispersed particles is very small in both the riser and the downer, indicating that dispersed particles pass through the system in a near plug flow pattern. Dispersion due to particle clusters in the riser, on the other hand, is very significant, contributing to the large axial solids backmixing and the bimodal solids residence time distribution in the riser.